In the field of stripline filter design, it is commonplace to employ a filter scheme as generally shown in FIG. 1. FIG. 1 depicts a filter 100 having three resonators 102, 104 and 106, an Input and an Output. Although the filter 100 is depicted as having three resonators, the filter 100 could possess any number of resonators, in principle. An input signal 108 propagates along an input transmission path (not shown) toward and coupled to the first resonator 102. If the input signal 108 contains energy in frequency ranges falling primarily outside of the range of frequencies, or passband, close to the resonant frequency of the first resonator 102, the signal 108 is substantially reflected so that it travels backwards along the input transmission path (not shown). The passband is controlled by adjusting the external and internal couplings to the resonator. If, on the other hand, the input signal 108 contains energy in frequency ranges falling primarily within the passband frequencies, an electromagnetic resonance is established within the first resonator 102. The electromagnetic resonance established within the first resonator 102- causes an electromagnetic wave to be coupled to the second resonator 104. Once again, the signal 108 is either reflected from or established within the second resonator 104, depending upon whether it contains energy in frequency ranges falling primarily within the frequency range determined by the coupling between resonator 102 and the second resonator 104. The strength of the electromagnetic wave propagating to the second resonator 104 is a function of, among other variables, the distance between the first and second resonators 102 and 104. Generally, the closer together the first and second resonators 102 and 104, the greater the strength of the electromagnetic wave in the second resonator. Thus, the general scheme of such a filter is that an electromagnetic wave propagates from resonator to resonator as long as it is within the frequency range determined by the resonant frequency of each resonator and the couplings between the resonators, otherwise it is reflected backwards. The magnitude of the standing waves established in a particular resonator is a function of, among other variables, the distance between the particular resonator and the preceding resonator. Consequently, the width of the passband of the filter 100 as a whole is a function of the ability of each resonator 102, 104, 106 to impart energy to a successive resonator 102, 104, 106.
FIGS. 2A and 2B depict a scheme by which filters such as the filter 100 depicted in FIG. 1 are tuned. FIG. 2A depicts two simplified resonators 200 and 202 disposed atop a substrate 204. The substrate 204 may have a ground plane disposed on the surface opposite the surface upon which the resonators 200 and 202 are disposed. The substrate is dielectric, and may be made of alumina, Duroid® microwave laminate, magnesium oxide, sapphire, or lanthanum aluminate, or other suitable material. The resonators and propagation paths are conductive and may be made of copper or gold, or superconductive materials, such as niobium or niobium-tin, and oxide supereonductors such as YBCO. A conductive cover 206 encloses the substrate 204 and resonators 200 and 202 thereby containing the electromagnetic fields. Exemplary electric field lines are depicted in FIG. 2A. The electric field between each resonator 200 and 202 and the surrounding environment takes on a particular form when the resonator 200 and 202 carries an electromagnetic wave with a frequency at the resonators″ 200 and 202 resonant frequency. If the electric field is disturbed, the resonant frequency of each resonator 200 and 202 (and therefore the center frequency of the passband of the filter as a whole) is altered.
FIG. 2B depicts the impact of the introduction of tuning tips 208 and 210 through the metallic cover 206 into the interior of the filter holder. The substrate is illustrated by the designation 204. The tuning tips 208 and 210 may take on the form of treaded cylinders, which maybe brought into greater or lesser proximity of the resonators 200 and 202 by rotation thereof. The tuning tips 208 and 210 can be dielectric materials that have a permittivity that is greater than the permittivity of the air, or vacuum within the conductive cover 206. Consequently, the electric flux density throughout the tuning tips 208 and 210 is greater than that of the air or vacuum, surrounding it. The tuning tips 208 and 210 disturb the field, by drawing more of the field towards themselves. By bringing a tuning tip 208 or 210 into greater proximity of a resonator 200 or 202, a greater portion of the field surrounding the resonator is disturbed. As the field is disturbed, the resonant frequency of the resonator is disturbed as well. Thus, the filter as a whole may be tuned by bringing the tuning tips 208 and 210 into greater or lesser proximity of the resonators 200 and 202.
One particular drawback of such a scheme is that as the tuning tips 208 and 210 are adjusted for the sake of tuning the center frequency of the filter, the bandwidth of the filter changes as well. This occurs because as the tuning tips 208 and 210 are brought into greater proximity to the resonators 200 and 202, they draw a greater portion of the field through themselves, meaning that a lesser portion of the field is available for facilitating resonator-to-resonator interaction (this is true for the case where the tuning tips are made of dielectric material, and the resonator structure is such that inter-resonator coupling is achieved via electric fields, rather than magnetic fields). Since, as stated above, bandwidth of the filter is a function of the ability of each resonator to impart energy to a successive resonator, the bandwidth of the filter drops as the tuning tips are brought into proximity of the resonators. Of course, the tuning tips (or entire rod) may be made of a conductor or superconductor, and the structure of the resonators themselves may be such that inter-resonator coupling occurs via electric fields, magnetic fields, or a combination of the two. Thus, bringing a tuning tip into closer proximity to a resonator may cause the bandwidth to either increase or decrease, depending upon the design of the filter. In the specific instances shown herein, bandwidth is decreased when the tuning tip is brought into greater proximity to the resonators.
The aforementioned scheme exhibits another drawback. Various communication schemes demand various bandwidths. For example, some PCS schemes demand a bandwidth of 5 MHz, while others demand a bandwidth of 15 or 20 MHz. FIG. 3A depicts a first exemplary filter that has a bandwidth of 5 MHz, while FIGS. 3B and 3C depict exemplary filters having 15 and 20 MHz bandwidths, respectively. As follows from the foregoing discussion, and as is depicted in FIGS. 3A, 3B, and 3C, greater bandwidth is achieved by locating the resonators in closer proximity to one another. Unfortunately, because the tuning tips are to be located over the resonators, varying inter-resonator spacing means that a different conductive cover must be fabricated for each communication scheme. This is due to the tuning tips penetrating the conductive covers. In this physical arrangement, since the tuning tips must be located over the resonators, and if the resonators are located in different positions for different communication schemes, then the holes in the conductive cover—through which the tuning tips must pass—must be located in different physical areas of the conductive cover for varying schemes. It will be appreciated, however, that it is generally undesirable to require different conductive covers for each communication scheme (e.g., because the numbers of parts are proliferated and costs are raised).
As is evident from the foregoing, there exists a need for a scheme by which a substantially planar stripline, or microstrip, type filter may be tuned while minimizing impact on filter bandwidth. There also exists a need for a stripline type filter scheme that can exhibit varying bandwidths without altering the physical position of the resonators making up the filter.